Heat Signatures

Thermal imaging technology can be traced as far back as 1800, when German astronomer Sir William Herschel discovered “invisible light” beyond the red part of the visible spectrum. In 1880, American astronomer Samuel Langley detected a cow’s body heat from more than 1,000 feet away. Advances continued, and thermal imaging technology was developed for military applications in the 1940s, primarily to identify enemy forces in dark environments.

As the cost of the various available thermal imaging technologies decreased, the tools were adapted for nonmilitary applications, such as firefighting, search and rescue, security and surveillance, predictive maintenance of electrical and mechanical systems, and building envelope inspections. According to Andy Teich, CEO of FLIR Systems Inc., Wilsonville, Ore., recent innovations in miniaturization and production techniques have enabled the manufacture of thermal imaging detectors for a fraction of what they cost just a few years ago.

“Today, every electrical contractor can have a thermal imaging camera in their truck or toolbox,” he said.

Today’s technologiesAll thermal imaging technologies work on the same basic principle of detecting and displaying differences in thermal energy; thermal imagers themselves, however, fall into the two major categories of cooled and uncooled.

“Cooled system are so named because they must be cryogenically cooled below –238°F, which is most often done by devices called cryocoolers,” said Zach Haas, senior product manager for Milwaukee Electric Tool Corp., Brookfield, Wis.

Cooled detectors are generally more expensive and better suited for applications that require imaging objects at extremely long distances and where extremely small differences in heat must be detected.

“While the mechanical microcooler in these thermal cameras adds significant sensitivity and performance, it also adds a maintenance requirement of typically once every 10,000 operational hours,” Teich said.

Uncooled detectors use technology called microbolometers, Haas said. As the imager is aimed at a scene or object, the detector absorbs the infrared (IR) radiation and heats up. This heating up changes the electrical resistance of the detector, and the measurement of that change is used to create the image.

According to David Dorn, director of thermal imaging for Schneider Electric, Palatine, Ill., the advantage of the uncooled microbolometer is that it forms good thermal images at a reduced cost. In addition, because they have no moving parts, uncooled thermal imagers tend to have much longer service lives than cooled cameras under similar operating conditions.

“However, these detectors are not as sensitive as cooled imagers and cannot see as far,” he said.

IR film versus thermographyIR photographic film sees a different wavelength of energy than what is commonly referred to as IR thermography, according to Michael Stuart, senior product marketing manager for Fluke Corp., Everett, Wash.

“IR film sees in the 700-nanometer [nm] to 900-nm range of the electromagnetic spectrum and is on the border of visible light and the near-infrared area of the spectrum,” he said.

According to the Eastman Kodak Co., operating at this end of the spectrum requires the target object’s temperature to fall between 482°F and 932°F to emit enough energy to expose the IR film, providing that the exposure time is sufficient.

“This doesn’t make infrared film practical for such applications as electrical inspections, especially since there are many heat issues that fall under 482°F that an electrician could not afford to miss,” Teich said.

Another limitation is that IR film is not radiometrically calibrated, meaning it cannot be used to measure accurate temperatures.

Thermography, however, captures noncontact temperature measurements and senses temperatures within the long-wave IR spectrum, making them able to detect a much broader range of temperatures, from –4°F to 3,632°F. Thermography, according to Dorn, has become easier to use, is less expensive than IR film and provides images in real-time.

Passive versus active thermographyMost thermal images used for electrical and electromechanical inspection work today are passive systems. According to Stuart, they employ the use of optics and a sensor that together collect IR energy emitted from equipment and the environment. The imagers then convert that energy signature into a false-color image that can be seen on an LCD screen or in an image file. Passive thermography is what the majority of electricians typically perform by simply viewing the heat being generated by electrical components and substation equipment.

“This image is commonly known as a thermogram or, simply, a thermal image. It is a map of an object’s apparent heat signature,” he said.

Active thermography, however, involves the intentional and active heating of an object or object surface through methods such as electrical impulses, heat lamps, pulse lamps, radio frequency, etc. Then, a thermal imaging system watches how the applied heat travels through an object or material.

“This is a common practice for materials analysis using thermal imaging,” Stuart said.

Applications include some types of transportation and building infrastructure inspection work.

“With proper training, differences or changes in the internal structure of a material—due to defects, damage or age—can often be discovered and diagnosed with active thermography,” Stuart said.

Additional uses for electrical contractors include inspection of distribution systems for high-resistance connections; current overload issues; fuse problems; heat issues with bus ducts, motor controllers, motor and bearing inspections; some power quality issues; and component malfunctions and failures. Because nearly all electrical issues ultimately create a temperature difference that can be seen clearly with a thermal camera, electricians can use the technology to accurately measure targets and visualize where potential problems may be brewing, according to Teich.

“Some electrical contractors are now even partnering with building maintenance specialists and using thermal imaging to conduct whole building energy audits to help reduce phantom loads, vampiric current draw and unnecessary I2R [copper] losses on equipment,” Stuart said.A primary benefit of thermal imaging is that it enables electricians to discover problems that may not be visible to the naked eye.

“Many problems in the electrical field manifest themselves as an increase in temperature prior to failure or fire. A thermal camera points out exactly where the problem lies, rather than needing to hunt for it with a spot temperature gun,” FLIR Systems’ Teich said.

Thermal cameras also enable electrical contractors to capture an image and actually show the customer exactly what and where the problem is, facilitating faster repair decisions and documenting repair effectiveness. Other benefits beyond predictive maintenance include improved operational efficiency; safer, noncontact, and faster and more reliable inspections; inspections of hard-to reach or dangerous equipment; and proper monitoring of potentially dangerous situations until a fix can be scheduled.

Knowledge is powerMany contractors today use handheld units to take their thermal pictures, but the key parameters for IR images of temperature and emissivity—how effectively heat is emitted by the object—are more complicated.

“Contractors need to know the different emissivity of materials because that can significantly affect the accuracy of the camera’s measurement,” Dorn said, adding that fixed cameras are more accurate in helping determine long-term scheduling of preventative maintenance. “The fixed angle makes these cameras more accurate than handheld units in respect to temperature measurement.”

According to Haas, the most important thing electrical contractors need to know about thermal imaging technology is how to use it correctly.

“Thoroughly understanding the technology well enough to correctly capture thermal images and diagnose problems in a system requires significant training that covers a variety of topics, from the history of IR measurement and thermography to the basics of heat transfer theory, application-specific content for electrical and mechanical systems, building and roof inspections, how to properly use thermal imaging equipment, and correct methods for capturing and analyzing thermal images,” he said.

Stuart agreed that, even though thermal imagers are more durable, easier to use, and more affordable than ever, proper training and practice is still critical for a solid return on investment. Organizations such as The Snell Group or the Infrared Training Center (ITC) can assist contractors that are serious about using thermal imaging as part of their predictive maintenance program.

“Classes from the ITC include Thermography Levels 1 through 3 as well as application-specific classes that address the particular challenges electrical contractors face every day,” Teich said.

In addition, the American Society for Non-destructive Testing (ASNT) typically addresses formalized training for thermal imaging in North America under standard ASNT-SNT-TC-1A-2006. The National Fire Protection Association also offers relevant information on electrical inspection and best practices in NFPA 70B and NFPA 70E.

“In these applications, proper methods of inspection must be used, and proper personal protective equipment (PPE) must be worn. However, through the use of noncontact inspection methods, such as IR thermography, a lower level of PPE is required,” Stuart said.

With improvements continually emerging, including analytic capabilities that create alarms when high heat parameters are met and price points dropping to become comparable with high-end pan/tilt/zoom security cameras, thermal imagers are becoming more affordable and easier to use.

“As a result, more and more people are beginning to see the advantages of thermal imaging in more and more applications,” Haas said.